The present invention relates to a circuit arrangement for operating at least one discharge lamp with a first and a second input terminal for connecting a supply voltage, an inverter, which comprises at least one first switch and one second switch, which are coupled in series between the first and the second input terminal and between which a bridge center point is defined, a drive circuit for at least the first switch and the second switch with an input for receiving a control signal, and an apparatus for generating an auxiliary voltage. In this case, the auxiliary voltage comprises a first capacitor, a terminal for the provision of the auxiliary voltage, which terminal is coupled to a reference potential via the first capacitor, a two-state controller with a first input to which the control signal in inverted form is coupled, a second input, which is coupled to the terminal for the provision of the auxiliary voltage, and an output, a switch with a control electrode, a working electrode and a reference electrode, the control electrode being coupled to the output of the two-state controller, the working electrode being coupled to the terminal for the provision of the auxiliary voltage, and a nonreactive resistor. The invention moreover relates to a method for generating an auxiliary voltage in such a circuit arrangement.
A circuit arrangement of the generic type which is known from the prior art is shown in FIG. 1 to illustrate the problem on which the invention is based. Said figure shows a segment of an electronic ballast which is generally connected to an AC voltage system via a filter circuit, a rectifier circuit and a PFC (power factor correction) circuit. Said segment is fed by the so-called intermediate circuit voltage UZW, which is stabilized by means of a capacitor CUZW. The intermediate circuit voltage UZW in this case feeds a half-bridge circuit, which comprises a first switch S1 and a second switch S2, and is generally of the order of magnitude of 320 V. The half-bridge center point HM is coupled, via a lamp inductor L, to a discharge lamp La, with which a starting capacitor C1 is connected in parallel and which is coupled to a reference potential via a coupling capacitor CK. The circuit arrangement has a controller 10, which can be driven digitally via an interface 12, for example in accordance with the DALI standard. In the standby operating mode, i.e. when the inverter is switched off, the controller 10 requires a current supply of approximately 2 mA, and during normal operation, i.e. when the inverter is in operation, a current supply of approximately 30 mA. An “on” signal at the interface 12 results in a half-bridge driver circuit 14 coming into operation and driving the switches S1 and S2 in accordance with a default entry.
The interface evaluation performed by the controller 10 must be ready for use at any time, even in the “off” state of the output circuit 16, which comprises the inverter with the switches S1 and S2, the lamp inductor L and the lamp La together with the circuit, in order to be able to receive and evaluate a new “on” command, for example. For this purpose, it is necessary to always supply a voltage to the controller 10 even in the “off” state. In order thus to keep the interface 12 always in the ready state, standby losses occur, which are generally undesirable.
The known solution derives the standby current required for the controller 10 via a nonreactive resistor RF and a two-state controller SSD, which is controlled via a switch QISS, directly from the intermediate circuit voltage UZW. In this case, the control signal which is used for switching on the half-bridge driver 14 is supplied in inverted form to the two-state controller SSD, with the result that the two-state controller comes into operation when the half-bridge driver 14 is switched off. Thus, the controller 10 is no longer supplied with voltage via its operational supply circuit 18, with the operational supply circuit, by way of example, in this case comprising a capacitor C2 and two diodes D1 and D2, but via an auxiliary voltage VCC provided at a capacitor CVCC. An input 20 of the two-state controller SSD is used for measuring the voltage VCC. The current source ISS illustrated in FIG. 1 can be implemented by an integrated circuit, but in a very simplified form also by a nonreactive resistor. As shown in FIG. 1, the standby supply at the capacitor CVCC is only active when the output circuit has been switched off via the interface 12. The two-state controller SSD keeps the auxiliary voltage VCC across the current source ISS, which is connected to the switch QISS, constant by virtue of it varying the duty ratio depending on the current consumption and the level of the intermediate circuit voltage UZW. The standby power loss in this solution is approximately 0.5 to 1 W. The two-state-controlled current source required is advantageously already integrated in the case of a few commercially available half-bridge drivers.
One disadvantage with this known solution is the still undesirably high power loss in the standby operating mode.
One further disadvantage of this known solution consists in the fact that additional auxiliary voltage generation is required for the normal “on” operating mode. In this case this is implemented by the operational supply circuit 18, which is based on the principle of deriving this voltage capacitively at a suitable point from the output circuit 16.
Another circuit arrangement (not illustrated) solves the problem of an additional auxiliary voltage supply for the normal “on” operating mode by virtue of the fact that the circuit arrangement comprises a step-down converter, which generates a controlled auxiliary voltage. It allows auxiliary voltage generation not only in the standby operating mode, but also in the normal “on” operating mode, with it being possible for standby power losses of from 0.3 to 0.8 W to be achieved. The disadvantage consists in the fact that such a circuit arrangement is comparatively expensive and requires a large number of components.